1. Field of the Invention
This invention relates to a brushless (non-contact) linear servomotor capable of performing speed control and position control with extremely high accuracy, and more particularly to a non-contact system linear servomotor which directly linear-drives a controlled object and eliminates any contact.
The present invention is applicable typically to a driving apparatus including a drive mechanism for a pen block in automatic drafting equipment, such as a XY plotter, a drive mechanism for a pen block in a recording instrument, such as a XY recorder or a strip-chart recorder, or a feeding mechanism for a printing head in a printing machine, such as a line printer, these mechanisms performing a linear movement at a high speed and with a high accuracy
2. Description of the Prior Art
Conventionally, the aforesaid apparatus have most usually adopted to drive mechanism which employs a rotary type stepping motor or a rotary type servomotor to move the controlled object linearly through gears, rollers, pulleys and wires.
In a case of using such a mechanism, however, its obtainable performance has a fixed limit to be discussed below, so that, when an engineer has intended to use it beyond the limit, he has been obliged to suddenly confront a difficult condition regarding techniques and manufacturing costs.
The first limit concerns the physical accuracy of position, linearity and reproducibility. For example, gears always include a certain backlash and dead zone and pulleys include a slip. The longer the wire is, the worse its influence on the accuracy by its expansion and contraction. The rotary type stepping motor is not of a closed loop control (the so-called open loop control) so that an eccentricity of a gear has a delicate influence on the linearity and also a fatal problem may be created in that the motor when intended to run at high speed, is apt to cause a step-out in the transient state. While, in a case of using the rotary type servomotor, a linear potentiometer is often used at the point of action (the controlled object) to realize the closed loop control to thereby lessen the factor of accuracy deterioration, but the linear potentiometer anew causes the accuracy deterioration. The linear potentiometer now widely in use comprises resistors of a wound type or a conductive plastic type and contact brushes so that its accuracy depends largely on the non-linearity of the resistor, hysteresis and backlash of the brush, and wear of the resistor.
The second limit concerns the reliability such as the durability of lifetime. The aforesaid transmission system includes a force transmitting member using a frictional resistance as an intermedium, or often uses rotary members, such as rollers, for bending the wire, whereby it is not possible to avoid wear in each component which causes the deterioration thereof. Especially, there is a high probability of deterioration of the wire and breaking thereof. On the other hand, the rotary type servomotor is composed mainly of a DC motor with brushes, whereby its lifetime should be considered to have a fixed limit. Also, wear of resistors in the potentiometer has been one of the factors lowering the reliability.
The third limit concerns a problem of noises. The aforesaid conventional methods transmit a rotating force of the rotary type motor to the controlled object through various transmitting members to thereby linearly drive the object, resulting in generation of considerably larger noises at each component, especially resonant noises in the wire and rotation noises in the roller. Hence, the machinery is often significantly degraded.
Several proposals have been made to eliminate the aforesaid defects. For example, application of a linear stepping motor has been proposed, which is intended to directly drive the pen block and the like so that the aforesaid defect can be eliminated considerably in part, but it is difficult to complete the closed loop control because of the use of stepping motor, resulting in difficulties which remain at high-speed operation and high acceleration. Also, the linear stepping motor is suitable for the automatic drafting machine, such as XY plotter, but not suitable for an analog recorder, thereby being defective in the limited application range.